Such IGBTs may especially be used as switching devices in power applications where they are very advantageous since they combine the preferred characteristic of bipolar junction transistor and MOSFETs, namely low conduction losses in the on-state, especially in devices with larger blocking voltages, and the possibility to be quickly turned on and off. These excellent switching properties of the IGBTs have in the past years resulted in a replacement of other semiconductor switching devices by IGBTs in many applications, but a significant research has been carried out for optimizing the function of these devices since they have in spite of the characteristics previously mentioned some not neglectible drawbacks.
This type of semiconductor device is particularly well suited for high power applications, and the invention is therefore especially occupied with the problem of providing an IGBT having SiC as semiconductor material, since it would then be possible to benefit from the superior properties of SiC in comparison with, especially Si; namely the capability of SiC to function well under extreme conditions. SiC has a high thermal stability due to its large bandgap energy, such devices fabricated from the material are able to operate at high temperatures, namely up to 1000.degree. K. Furthermore, it has a high thermal conductivity, so that SiC devices may be arranged with a high density. SiC also has a more than five times higher breakdown field than Si so that it is well suited as a material in high power devices operating under conditions where high voltages may occur in the blocking state of a device. An IGBT made of this material has particularly low conduction losses in the on-state compared to a MOSFET. However, it is emphasized that the invention is not in any way restricted to this choice of semiconductor material, but any semiconductor material, such as Si, is conceivable.
Furthermore, such a trench does not have to be as deep as in normally so called trench IGBTs, but it may be considerably more shallow.
The major example of the drawbacks mentioned above of prior art IGBTs arises from the fact that an IGBT has three pn-junctions in series and thereby a thyristor-like structure, which results in problems with so called latchup in its on-state. This problem as well as other drawbacks related thereto, will now be explained while referring to FIG. 1 of the appended drawings which illustrates a prior art IGBT having superimposed: a drain 1'; a highly doped p-type substrate layer 2' for forming good ohmic contact to the drain; a highly doped n-type buffer layer 3'; a low doped n-type drift layer 4'; a p-type base layer 5', which preferably is moderately doped, but may also be differently doped in different regions thereof; a highly doped n-type source region layer 6'; and a source 7'. The device also has a trench 8' carried out in the base layer 5', and the walls and the bottom of the trench are covered with an insulating layer 9' and a gate electrode 10 on top thereof.
By applying a voltage above a threshold voltage value to the gate electrode 10' a conducting inversion channel may be created at the interface between the base layer 5' and the insulating layer 9' for the conduction of electrons from the source region layer 7' to the drift layer 4' thereby turning the device on. The device may very quickly be turned off by cutting off the voltage supply to the gate electrode. This is the normal operation of the IGBT, but this function is only there in a so-called safe operating area (SOA), and outside this area the following mechanism will appear.
Since an IGBT has a highly doped substrate layer 2', the electron current (indicated by arrow 11') flowing through the inversion channel and toward the drain causes a substantial hole injection from the substrate layer 2' into the drift layer 4'. The holes move across the drift layer taking a variety of paths 12', and reach the base layer 5', and they will also move toward the source 7' for recombining with electrons from the source. Such hole paths 12' will go around the lower trench corner 13', since the holes are attracted by the negative charge of the electrons in the inversion channel thereby resulting in an electrical field concentration there, which may, when it is high enough, cause the insulating layer to burn there and the device to be destroyed. Furthermore, the paths 12' will accordingly extend laterally through the base layer below the highly doped n-type source region layer 6' and will feel the lateral spreading resistance indicated at 14' of the base layer. This in turn results in a lateral voltage drop in the base layer along the junction between the base layer 5' and the source region layer 6'. This tends to forward bias the junction, and if the voltage is large enough, substantial injection of electrons from the source region layer into the base layer will occur and the parasitic thyristor composed of the two parasitic npn and pnp transistors created this way will latch on and a latchup of the IGBT will thereby have occurred. Once the IGBT is in a latchup state, the gate no longer has any control of the drain current. The IGBT may then only be turned off in the same way as a conventional thyristor. If the latchup is not terminated quickly, the IGBT will be destroyed by excessive power dissipation. There is a critical value of drain current which will cause a large enough lateral voltage drop to activate the thyristor.
Attempts have been made to tackle these problems, and a trench-IGBT according to the introduction has been described by Constapel, Korec and Baliga in "Proceedings of 1995 International Symposium on Power Semiconductor Devices and ICs, Yokohama". It also described that IGBT has an additional, highly doped, p-type layer located at the bottom of the trench in the drift layer, next to the insulating layer separating the drift layer from the gate electrode. The holes otherwise injected into the base layer during forward conduction and going under the source region layer and thereby causing a lateral voltage drop will now instead, or in any case a larger fraction thereof, enter the highly doped additional layer and be diverted through a separate diverting circuit to the source of the device. As a result, a higher drain current may be accepted than in the prior art devices according to FIG. 1 and the safe operating area may be increased. However, it is a disadvantage of this device that a separate diverting circuit has to be applied for the holes. Furthermore, it would, of course, be desirable to extend the safe operating area of such an IGBT further.